If you want to break your output into smaller groups, you can specify multiple
column names or expressions in the GROUP BY clause. Output in each group must satisfy
a specific combination of the expressions listed in the GROUP BY clause. The more
columns or expressions entered in the GROUP BY clause, the smaller the groups will be.

The tutorial exercise below shows you how to break data into groups per "tag" and per
year when they were created. Then the group function COUNT(*) is applied on each group: